in sandigen Watten Norma Angelica Hernandez-Guevara

Distribution and Mobility of Juvenile Polychaeta in a Sedimentary Tidal Environment

Verbreitung und Mobilitä juveniler Polychaeten in sandigen Watten

Norma Angelica Hernandez-Guevara

Ber. Polarforsch. Meeresforsch. 502 (2005) ISSN 1618

-

3193

Norrna Angklica Hernandez-Guevara Wattenmeerstation Sylt

Alfred-Wegener-Institut fü Polar- und Meeresforschung Hafenstr. 43

25992 LisUSyIt Germany

Die vorliegende Arbeit ist die inhaltlich unverändert Fassung einer Dissertation, die 2004 im Fachbereich Zoologie der Universitä Kiel vorgelegt wurde.

Contents

Zusammenfassung

Summary

1. General Introduction

2. Abundance and distribution of juvenile and adult polychaetes:

are tidal flats nursery habitats? 11

3. Small-scale dispersion of juvenile polychaetes:

indirect evidence of benthic mobility

4. Nobility of marine benthic invertebrates: an experimental

approach with juvenile polychaetes On a sandy tidal flat 7 2

5. Final discussion 89

References 96

Acknowledgments 105

Es wird traditionell angenommenI dass die räumlich Verteilung benthischer Populationen hauptsächlic von pre-settlement (vor der Ansiedlung) Prozessen bestimmt wird. Doch häufe sich die Indizienl dass auch viele post-setflement (nach der Ansiedlung) Prozesse eine wichtige Rolle spielen können I n dieser Studie wurde die Ausbreitung juveniler Stadien nach der Ansiedlung als ein mögliche Schlüsselereigni fü Verteilungsmuster von Adultpopulationen untersucht. Da Borstenwürme (Polychaeten) eine der vielfältigste und zahlreichsten Tiergruppen in marinen Weichböde sindl wurden sie als Beispiel herangezogen. Zuerst wurden Verteilungsmuster von juvenilen und adulten Polychaeten im Sylter Wattenmeer verglichen. Eine Beprobung erfolgte in fün Habitaten: Sandwatten! Seegraswiesen! Schli~kgrasbulten~ Muschelschillfeldern und Miesmuschelbänken Insgesamt wurden 43 Polychaetenarten bestimmt. Acht Arten trugen mit mehr als 90Y0 zu der Gesamtabundanz bei (Scoloplos armigec wgospio elegansl Nereis virensI Capitella capitatal Microphthalmus s P. Exogone naidnal Spio martinensis und Phyllodoce mucosa)

.

Der An teil der übrige Arten an der Geamtabundanz betrug jeweils weniger als 1°/o Juvenile waren signifikant zahlreicher in strukturierten Habitaten (Seegraswiesen und Muscheischiilfeldern) als in nicht-strukturierten Habitaten (Sandwatten). Strukturierte Habitate könnte zlso als Kinderstube dienen. Eine räumlich Trennung von juvenilen und adulten Würmer durch verschiedene Habitate wurde fü Ophela rathkeiI Microphthalmus sps und Phyllodoce mucosa festgestellt. Das läss vermutenI dass in diesen Populationen Wanderungen nach der Ansiedlung stattfinden könnten Ein weiteres Anzeichen fü eine räumlich Verbreitung durch früh benthische Entwicklungsstadien fand sich bei nahezu tägliche Beprobung von Verteilungsmustern juveniler Würme auf kleinräumige Skala (4 m2) übe einen Zeitraum von zwei Monaten. I n diesen zeitlich und räumlic hoch aufgelöste Verteilungsmustern zeigten Spio martinensisI Typsyllis hyalina, Ophelia rathkei und Capitella minima eine hohe Variabilität die größtentei auf fortlaufende Ein- und Auswanderungen zurückzuführ sind. Sowohl aktive wanderung als auch

passive Drift könne dabei eine Rolle spielen. Anders als bei driftenden juvenilen Muscheln, wurden nur wenige juvenile Würme in Wasserproben und bodennahen Driftnetzen gefunden. Um zu untersuchen, ob juvenile Polychaeten entweder im Boden bzw. im Übergangsbereic Wasser-Sediment kriechen oder in der bodennahen Wassersäul schwimmen, wurde ein in sit~Experiment durchgeführt Hierfü wurde eine Kombination von Driftnetzen und in den Boden eingesenkten, nach oben abgedeckten Rinnen verwendet. Die Rinnen wurden mit von Polychaeten befreitem Sediment gefüllt und übe drei aufeinander folgende Gezeiten exponiert. I n den Driftnetzen wurden keine juvenilen Würme gefunden.

I n den Rinnen hingegen wurden Ophelia rathkei, Pygospio elegans und Typosyllis hyalina registriert, die eine Strecke von wenigstens einem Meter pro Tag aktiv innerhalb der Rinnen kriechen konnten. Capitella capitata erwies sich als weniger mobil. Es wird angenommen, dass eine benthische Ausbreitung der Juvenilen nach der Erstansiedlung (post-settiement-Verbreitung) fü die Populationsökologi einiger Polychaeten eine wichtige Rolle spielt. Junge Würme siedeln sich dort an, wo fü sie günstigst Bedingungen herrschen, unabhängi von den Habitaten, die von Adulten bevorzugt werden. Aktive, benthische Wanderungen bringen die jungen Würme zur richtigen Zeit zu geeigneten Orten, wo sie ihren Lebenszyklus vollenden können

I t has been assumed that the distribution of marine benthic populations depends mainly On pre-settlement processes. However, evidence for many post-settlement processes has been provided recently. Juvenile dispersal after settlement is here investigated whether it attains a key role in determining spatial patterns of adult populations. Since polychaetes are one of the most diverse and abundant taxonomic groups in soft-sediment environments, they are chosen to explore the importance of this process. The first step in this study was to compare the distribution patterns of juvenile and adult polychaetes in a sedimentary tidal area of the Wadden Sea near the Island Sylt. Sampling included sandy flats, seagrass beds, cordgrass patches, mussel beds and fragmented shell patches. A total of 43 polychaete species is recorded, Eight species together comprised more than 90%

of total a bu nda nce (Scoloplos armiger, Pygospio elegans, Nereis virens, Capitella capitata, Microphthalmus s p

. ,

Exogone naidina, Spio rnariinensis an d Phyllodoce mucosa). All other species contributed less than 1%. Juvenile abundance was significantly higher in structured habitats (Seagrass beds and fragmented shell patches) than in non-structured ones (sandy flats). Structured habitats could serve as nurseries. Spatial separation of juveniles from adults across habitats was found in Ophelia rathkei, Microphthalmus s p

.

a nd Phyllodoce mucosa. Th is may indica te juvenile migration as a mandatory process in such populations. A second indication of dispersal by juvenile benthic stages was found, when small-scale distribution patterns (4 m2) were analyzed almost daily over a two months period.

At high spatio-temporal resolution, distribution patterns for Spio martinensis, Typosyllis hyalina, Ophelia rathkei a nd Capitella minima exh i bi ted a high va ria bi lity explained mostly by ongoing Immigration and emigration. Both processes, active migration and passive transport may play a role. I n contrast to extensive drifting in juvenile bivalves, only few juvenile polychaetes were found in water samples or in nets above the bottom. With the hypothesis that juveniles crawl at or below the sediment-water interface, an in situ experiment was set up. A combination of drift

nets and covered grooves placed level with the sediment surface were used in order to trace crawling performance in juveniles. Grooves were filled with sediment free of polychaetes and placed On intertidal flats for three tidal cycles.

No juvenile worms were found in the drift nets, while in the grooves Ophelia rathkei, Pygospio elegans, and Typosyllis hyalina were able to crawl actively at least one meter per day, while Capitella capitata was less mobile. I t is assumed that post-settlement dispersal plays an important role in the population ecology of some polychaetes. Juveniles are able to settle in habitats favorable specifically for juveniles irrespective of habitats preferred by adults. Active migrations a t the bottom may bring juveniles in due time to sites suitable for the completion o f their life cycle.

An understanding of the patterns of distribution and abundance in organisms is often the basis for ecological evaluations and management decisions (Andrew and Mapstone 1987). An important task for marine ecologists concerns the knowledge of processes that regulate these patterns in benthic communities (Valiela 1984).

Patterns are generated by a combination of physical forces and biological interactions. Sediment type, waves action, erosive currents and light intensity are examples of physical factors which limit benthic populations; biological ones include dispersal potential, intra- and interspecific competition, predation and parasitism.

I n general benthic marine invertebrates release propagules that either remain near their parents or disperse as planktonic larvae. The latter may reach distant destinations more or less favorable for settlement and metamorphosis (Fraschetti e t al, 2003). The structures of marine benthic populations with a pelago-benthic life cycle arise from pre- and post-sefflement processes (Fig. 1; Possingham and Roughgarden 1990).

The relative importance of pre- and post-settlement processes differs between localities, hard and soft-bottoms as well as species (Stoner 1990, 0lafsson

et

^al.

1994, Hunt und Scheibling 1997, Todd 1998, Fraschetti et al, 2003). Usually planktonic larvae are considered to serve as the dispersal phase of the population (Strathmann 1974, Scheltema 1986). Planktonic larvae are also considered the most vulnerable stage in the life cycle in marine invertebrates, since larval mortality exceeds 90°/ (Thorson 1950). Nevertheless, i t is discussed whether benthic distribution patterns are predictably based On the fate of larvae (Bhaud 1982, 1998 and 2000, Todd 1998).

Primary dispersal

-3

Secondary dispersai

01 (D D.

CL Â

Adult Settlement Adult

population Metamorphosis frorn

pelagic t o benthic life population

Post-settlernent processes

1

Juvenile dispersal

1

Figure 1. Scherne of pre- and post-settlement with primary and secondary dispersal in benthic invertebrates with pelago-benthic developrnent. By means o f secondary dispersal juveniles may either return t o the parent population or found an adult population at a new site. The modes and importance of secondary dispersal in juvenile polychaetes is the subject of this study (bold signs).

Settlement is a critical process (see Woodin 1986, and Butman 1987 for reviews).

However, a clear definition is still pending. Butman (1987) considers settlement as the moment when the organism adopts a behavior which is indicative of the benthic life history stage. The site of settlement is critical for the success of recruitment. While settlement is a biological phenomenon, recruitment is operationally defined as the entry into the benthic population of individuals that have survived up to a specific size after settlement (Fraschetti et al. 2003).

Recruitment has five major components: input of propagules into the water column, their transport, planktonic mortality, settlement and post-settlement growth and survival (Jenkins etal. 1999).

Post-settlement events include a wide spectrum of processes, from adult-juvenile interactions (chemical cues, bioturbation, competition for space, etc.) to predation, interspecific competition and performance in the vagaries of the physical environment (~lafsson et al. 1994, Todd 1998, Fraschetti et al. 2003). Early juvenile mortality may rival the loss of larvae as the most important factor influencing benthic populations, since in some cases mortality is higher than 9O0/0 of the larvae that have settled. Factors involved in juvenile mortality after settlement were reviewed in Gosselin and Qian (1997) and Hunt and Scheibling (1997). Particularly studies On colonization or recolonization of disturbed areas emphasized the importance of post-settlement Stages (juveniles and adults) to disperse into a vacated area (Bonsdorff 1983, Levin and DiBacco 1995, Whitlatch et al. 1998). For bivalves a secondary dispersal phase (also called bysso-pelagic migration phase, Bayne 1964) is well documented (Armonies 1992, Armonies and Hellwig-Armonies 1992, Lasiak and Barnard 1995, Dunn et al. 1999, Norkko et al.

2001). Secondary dispersal may be passive (resuspension, drift), active (swim, crawl), or a combination of both, and takes place in the sediment-water interface or the water column (Günthe 1992). Post-settlement events generally operate at smaller spatial scales than pre-settlement ones (Fraschetti etal. 2003).

The objective of this study is to explore post-settlement dispersal in polychaete worms. Polychaetes are often the most abundant or second most abundant after bivalves in the marine macrobenthic fauna in the Wadden Sea (Beukema 1989, Lackschewitz and Reise 1998). They are also one of the groups with the highest diversity of reproductive traits among marine invertebrates (Giangrande 1997).

This is probably due to the relative simplicity of their reproductive Systems combined with a high plasticity and adaptability to different habitats (Wilson 1991).

A study of Rodriguez-Valencia (2003) has dealt with the planktonic part of the polychaete community in the List tidal basin between the islands of Sylt and R0m0 in the northern Wadden Sea. He describes larval distribution patterns and factors that could affect larval ecology. This companion study deals with the benthic component. Three main questions are treated:

How are early post-settled polychaetes distributed in space and time in relation to the adult distribution? (Chapter I1 Abundante and distribution of juvenile and adult polychaetes: are t/dal flats nurmy habitats,?

What evidence exists that polychaete post-settlers have the ability of secondary dispersal? (Chapter I11 Small-scafe dispersion of juvenile polychaetes: indirect evidence of benhhic m o b i l i .

Can this secondary dispersal be traced either in the water column or in the sedimen t? (Cha pter I V Mobility of marine benthic inverhebrates: an experimental approach with juvenile polychaetes on a sandy tidaf fla f )

Finally chapter V (Final discussions) gives an overview on the ecological implications of the results found during this study. The importance of some post- settlement events for benthic-pelagic coupling is briefly discussed.

2. ABUNDANCE AND DISTRIBUTION OF JUVENILE AND ADULT POLYCHAETES: ARE TIDAL FLATS NURSERY HABITATS?

Abstract

I n marine benthic populations, juvenile benthic stages may be found outside the area of their adults. Since pelagic larvae may disperse over wide distances, it is possible that settlement occurs far away from their source population and in different habitats. Are juveniles after settlement capable to return to source areas, or the habitat types of the adults, or do they remain at the sites of settlement? I n a tidal basin of the northern Wadden Sea (North Sea) where sandy tidal flats dominate, distribution patterns of juvenile and adult polychaetes were compared.

Since structured habitats may provide protection against wave disturbance or predators, high abundance of juveniles was expected in such habitats. Samples were taken in seagrass beds, cordgrass patches, mussel beds, and fragmented shell patches as well as in the more extensive sandy flats (as non-structured habitat) at intertidal and subtidal sites. Juvenile stages were found in 10 out of 43 polychaete species. For Eulalia v i M s juveniles were recorded only. Both, juveniles and adults showed a preference for the structured seagrass beds and fragmented shell patches. Cordgrass patches and mussel beds were not suitable habitats for polychaetes. High densities of juvenile polychaetes in seagrass beds and fragmented shell patches Stress the role of these structured habitats as potential nurseries. This implies that juveniles may undertake migrations to reach the unstructured area where the bulk of their adults reside.

1. Introduction

The description of patterns is of fundamental importance in ecology and the Information on the distribution and abundance of organisms is often the sole basis

for management decisions (Andrew and Mapstone 1987). There is still only a modest understanding on how patterns in marine benthic invertebrates are maintained (Snelgrove and Butman 1994, Snelgrove etal, 2001).

Spatial patterns in soft sediment assemblages from temperate regions have primarily been correlated with changes in water depth and sediment characteristics (Gray 1974, 1981; Whitlatch 1981). Early post-settled organisms were often ignored in these studies. I n invertebrates with a bentho-pelagic life cycle these post-settlers represent only a temporary part of the population. Also, methods were often size-selective and the small juveniles were overlooked. Most studies on post-settled organisms focused on recruitment rather than initial colonization, and where experimental sediments were spatially separated from the natural habitat (non in-situ approaches) migration potential was disregarded (Snelgrove etal, 2001).

The "specific-area" a term introduced by Bhaud (2000), is defined as that area where larvae can settle, juveniles can grow, and adults can reach maturity and reproduce. Thus, records of species occurrence not including the whole life cycle are insufficient (Bhaud 2000). Some habitats like seagrass beds may play a very important role in the distribution of some benthic species, because they could act as nursery habitats (Bostrom and Bonsdorff 2000). I n the selected study area, List tidal basin, an overview on macrobenthic abundances is given by Reise and Lackschewitz (1998). Studies On specific habitats and their associated biota are available for seagrass beds (Schanz 2003), mussel beds (Buschbaum 2002, Saier 2002), fragmented shell patches (Wolf 2002), cordgrass patches (Löb 2002).

Specific studies of polychaete distribution patterns are scarce (Reise 1983a and b, 1984; Reise et al. 1994, Zühlk and Reise 1994). The main questions to be answered in this study are: How are benthic polychaete stages distributed in the List tidal basin? Have early post-settled worms different distributions than adults?

Do juveniles prefer structured over non-structured habitats?

It could be expected that juveniles have higher abundances in structured habitats because these offer protection against wave disturbance, predation, or provide specific food (Beck etaL 2003).

2. Methodology 2.1 Study Area

This study was carried out at the northernmost part of the German Wadden Sea (North Sea) in the List tidal basin (Fig 1). This basin was formed about 5,500 years ago and became confined by causeways constructed at the first half of the 20^ century (Bayerl and Kostner 1998). This bight comprises about 400 km2; with one third being intertidal flats. Sandy sediments predominate, and 3% of the tidal area consists of muddy flats and 2% of salt marshes (Bayerl e t al. 1998). Mean tidal range is 2 m. Tides are semidiurnal, and the high tide water volume is twice the low tide volume (Backhaus e t al. 1998). Salinity remains close to 30 PSU and water temperature in summer rarely exceeds 22OC (Asmus 1982). The tidal inlet 'Lister deep" (2.5 km wide) is the only connection with the North Sea and it transports about 7 X 1 0 m3 of water during each tide.

Diverse habitats are represented in the List tidal basin: sand flats, mud flats, mussel beds, seagrass beds (Zostera noltii and Z manna), fragmented shell patches, and cordgrass patches (Spartina angkca). Gätj and Reise (1998) provide a detailed description of the hydrodynamic and biotic characteristics of the List tidal basin.

2.2 Materials and Methods 2.2.1 Sampling Methods

Samples were taken at several sites of the List tidal basin and one site is located south of Hindenburgdamm (See Fig 1, Table 1). A detailed description of sampling methods and design is given below.

Table 1. Sample sites with dates and habitats. Numbers in ( ) indicate depth (m) above (+) and below (-) mean tide level. For locations See Fig. 1.

I= Intertidal Zone, S= Subtidal Zone Site (location in study

area map) Ostfeuerwatt (A)

Möwenbergwat (B)

Oddewatt (C)

Lister Ley (D)

Uthör (E) Blidsel (F)

Hunningen Sande (G)

Rantum (H)

Date of sampling 2000: Apr 28, May 12, Jun 9

2001: Mar 11 and 31, Apr 16, Aug 1 and 15 2002: Aug 1

2000: Apr 20, May 8 and 15, Jun 2, Aug 16 2001: Mar 10 and 30, Apr 15, Jul 24, Aug 22 2002: Aug 12

2001: Jul 24, Aug 14 2002: Aug 14

2001: Jul 13 and 27, Aug 10

2002: Aug 20

2001: Mar 10 and 30, Apr 15

2001: Aug 3

2000: May 15, Jul 17, Aug 22

2001: Mar 5, May 14, Jun 18

2001: Aug 7 and 20

Habitats

- -

Sandy flats (+0.5)

Sandy flats (+0.5) Mussel beds (-0.5)

Sandy flats (-1.0) Fragmented shell (-1.5) Mussel beds (-1.5) Sandy flats (-3.5)

Sandy flats (+0.5) Sandy flats (+0.5)

Cordgrass patches (+0.75) Seagrass beds (-1.0) Sandy flats (-4.0) Fragmented shell (-4.0)

Sandy flats (+0.5)

Cordgrass patches (+0.75) Seagrass beds (-0.5)

Figure 1. Study area in the northern Wadden Sea. Spring low tide is stippled. For sampling locations A-H See Table 1.

I n order to determine abundance of juvenile and adult Polychaeta in different habitats, samples from sandy flats, seagrass beds, cordgrass patches, mussel beds, and fragmented shell patches were collected.

Each sample consisted of a sediment box corer 15 X 15 cm (0.0225 m2) down to a sediment depth of Ca. 20 cm. From each core, three sub-samples (10 cm2 X 5 cm depth) were obtained. After that, the core was sectioned as follows: the first 5 cm were sieved through 500 pm mesh and the rest (aprox. 15 cm) was sieved through a 1000 pm mesh while the retained material was taken to the laboratory for analysis. Sub-samples were sieved through a 250 Pm mesh in the laboratory (Fig. 2).

Sampling design Intertidal sites:

At the intertidal sites if only one habitat type was present (e.g. Ostfeuerwatt with only sandy flats), 6 samples (box corer 15 X 15 cm) were collected along a transect (ca. 500 m) parallel the mean water line with a 100 m interval between samples. Where more than one habitat was present (e.g. Blidsel with seagrass beds, Spart/na anglica patches and sandy flats), one transect (ca. 300 m) was defined along each habitat and 4 to 6 samples were taken (Fig. 2).

Subtidal sites:

I n the subtidal Zone near of Lister Ley, samples were taken at 8 sites along a transect. At each sample site three box cores (0.02 m2) were collected.

At Hunningen Sande, samples were taken at three sites, and also at each site three box cores were collected (Fig. 2).

Intertidal

3 subsarnples each 50

3 (through 250p)

Rest through 500p and

lOO0p

14 X 14 crn

Subtidal

Figure 2. Schematic description of sarnpling methods applied at intertidal and subtidal zones.

Adult polychaete stages were identified with Hartmann-Schröde (1996), larval stages with Bhaud and Cazaux (1992). For juvenile stages no identification keys are available. They were identified with adult and larval identification keys, since a combination of morphological characteristics of both developmental stages were present in juveniles. A reliable determination was only possible with both identification keys and additional literature on specific taxa. Dr. Angel de Leon- Gonzalez (University on Nuevo Leon, Mexico) confirmed the identifications.

The number of organisms found as well as their developmental stage was recorded. Here, larvae were considered as organisms having characteristics for planktonic life (pigments, yolk reserves, swimming Organ, cilia, etc.; Bhaud and Cazaux 1992) but found alive in the sediment and without traces of metamorphosis. Juvenile stages were defined as organisms that have already metamorphosed but lacking size and morphology known from reproductive individuals. Adults were sexually mature organisms and which show conspicuous adult characteristics. Criteria used in specific cases are presented in Table 2.

Table 2. Distinguishing characteristics for juvenile and adult polychaetes of the most frequent species found in the study area.

Scoloplos armiger Pygospio elegans

Polydora ciliata Polydora cornuta Capitella capitata

Capitella minima Phyllodoce mucosa

Lanice conchilega Microphthalmus s p

.

Nereis virens Nereis diversicolor Typosyllis hyalina

Juveniles

Size: 0.2 mm- 15 mm Presence of melanophores, swim cilia and setae

Size: 0.2 mm-10 mm Transparent

Size: 0.2 mm- 10 mm Size: 0.2 mm- 30 mrn Transparent coelome Size: 0.2 mm- 30 mm Size: 0.2 mm- 5 mm Transparent

Size: 1- 30 mm Size: 0.2 mm-10 mrn

Adults

Size: > 15 mm

Without larval pigments (melanophores)

Size: >10 mm Size: >30 mm Size: >30 mm Size: >5 mm Size: >30 mm

2.2.2 Habitat preferences

Comparisons of habitat preferences were done with samples taken in August 2001. For this section all species are considered.

To test if structured habitats were preferred over a non-structured habitat, seagrass beds, cordgrass patches and sandy flats were compared at the Same locality. Also mussel beds, fragmented shell patches and sandy flats were compared at a single locality.

I n order to test for differences in abundance or species composition within intertidal sandy flats, comparisons between sites in Königshafe (Fig. 1: A, B, C) were made.

Comparison between sub- and intertidal habitats was done for intertidal sandy flats Konigshafen (Fig. 1: Al B, C) and a subtidal site in Lister Ley.

All comparisons were made with One-way ANOVA-tests using STATISTICA for Windows Version 6 [Stat Soft, Inc. (2003)l. I n case of significant differences, post- hoc teste (Tukey HSD test) were made.

Comparison of species composition in sandy flats was made by means of Cluster analyses using PRIMER 5 for Windows Version 5.2.9.

2.2.3 Species-specific patterns

For the most abundant species teste on habitat preferences were made as described above.

I n order to test whether adult and juveniles are distributed in the Same way, analyses of abundances over time were made for 2001 due to a high temporary resolution of sampling in that year. Mean abundances of juveniles and adults per sampling month were obtained lumping all subtidal sites and all intertidal sites near Königshafe (Fig. 1: A-D) together.

3. Results

3.1 Species spectrurn

Eighteen polychaete farnilies were found in benthic stages, 29 genera and 43 species. 42 were found as adult stages, 31 as juveniles and 4 also as larvae (Table 3). Spionidae were the most diverse with 11 species, followed by Nephtyidae (6 species) and Phyllodocidae (4 species). Eulalia viridis was only found in juvenile stages. For Magelona mirabilis, Nephtys hombergii, N. longosetosa, N. caeca, Phyllodoce maculata, Protodrilus adhaerens, Sphaerodoropsis baltica, Scolelepis foliosa, Spio filicornis, and Spiophanes bombyx no juvenile stages were found.

Table 3. List of identified polychaete species in the benthos near the island of Sylt (northern Wadden Sea) frorn 2000 to 2002. For locations See Figure 1 and Table 1. Letters in

[I

indicate the development stages found.

L=larvae, J=juvenile, A=adult.

--- -

--- -

-

^{--" ma}

- --

^{m-7 -"-}

- - ---"---

Family _{-- - - ---} Species _{- -- - -- --- -- -} Location Phyllodocidae Phyllodoce (Anaitides) maculata (LINNE, 1767) [ A l D OERSTED, 1843 Phyllodoce (A.) mucosa OERSTED, 1843 [A, J] ABCDEFGH

Eteone (Eteone) longa (FABRICIUS, 1780) [ A, J] ABCDEFGH

Eulalia viridis (LINNE, 1767) [J] D

Hesionidae Microphthalmus sp. (WEBSTER & BENEDICT, 1887) [AI J] ABCDEFGH MALMGREN, 1867

Syl li dae Typosyllis (Typsyllis) hyalina (GRUBE, 1863) [AI J] ABCDEG GRUBE, 1850 Exogone (Exogone) naidina OERSTED, 1845 [A, J] ABCDEG Nereididae Nereis (Hediste) diversicolor0.F. MULLER, 1776 [AI 31 ABCFH JOHNSTON, 1865 Nereis(Neanthes) virensS~~s, 1835 [A, J] ABCDEFGH

Ne ph tyidae Nephtys caeca (FABRICIUS, 1780) [ A l DG GRUBE, 1850 Nephtys cirrosa EHLERS, 1868 [AI J] DG

Nephtys hombergii SAVIGNY, 1818 [ A l ABCDG

Nephtys incisa MALMGREN, 1865 [ A l G

Nephtys longosetosa OERSTED, 1842 [ A l CDG Nephtys pulchra RAIN ER 199 1 [Afã3J__&

---=*--

- ---

L='-"'

-

^D

Table 3. Continued.

0 r bi ni idae Scoloplos (Scoloplos) armiger (0. F. MÃ LLER, 1776) ABCDEFGH HARTMAN, 1942 [AI J]

Spioni dae Malacoceros fuliginosus (CLAPAREDE, 1868) [Al J]

GRUBE, 1850 Polydora (Polydora) ciliata (JOHNSTON, 1838) [AI J, L]

Polydora (Polydora) cornuta Bosc, 1802 [Al J]

Polydora s P.

Pseudopolydora s P.

Pygospio elegans CLAPAREDE, 1863 [Al J, L]

Scolelepis (Scolelepis) foliosa (AUDOUIN & MILNE- EDWARDS, 1833) [ A l

Scolelepis (S.) squamata (0. M. M U LLER, 1806) [AI J]

Spio filicornis (0. F. MULLER, 1766) [ A l Spio martinensis MESNIL, 1896 [Al J, L]

Spiophanes bombyx ( C L A P A R ~ 1870) [ A l Magelonidae Magelona alleni WILSON, 1958 [AI J]

CUNNINGHAM & Magellona mirabilis (JOHNSTON, 1865) [ A l RAMAGE, 1888

ABCDFG BCDG ABCD ABFH

B ABCDEFGH

BD ADG BDG ABCDEGH

BCDGH G BDG

Protrod ril idae Protodrilus adhaerens JAGERSTEN, 1952 [ A l DG CZERNIAVSKY, 1881

Pa raon idae Aricidea (Aricidea) minuta SOUTHWARD, 1956 [AI J] ABCDEG CERRUTI, 1909

Cirratulidae Aphelochaeta marioni (SAINT-JOSEPH, 1894) [A, J] CDF CARUS, 1863 Tharyx killariensis (SOUTHERN, 1914) [AI J] A

Opheliidae Ophelia limacina (RATHKE, 1843) [AI J]

MALMGREN, 1867 Ophelia rathkei MCINTOSH, 1908 [Al 31

G ABCDEGH Ca pitel lidae Capitella capitata (FABRICIUS, 1780) [AI J] ABCDEFGH GRUBE; 1862 Capitella minima LANGERHANS, 1880 [AI J] AC

Heteromastus filiformis (CLAPAREDE, 1864) [A, J] ABCDEFH Arenicolidae Arenicola marina (LINNE, 1758) [AI J]

JOHNSTON, 1846

ABCDEFH

Table 3. Continued.

--- ----

^-"

--- ---

^{-----=P- P"-}

Pectinariidae Pectinaria (Lagis) koreni MALMGREN, 1865 [AI J] BDH QUATREFAGES,

1865

Terebellidae Lanice conchilega (PALLAS, 1766) [AI Jf L] BCDEGH MALMGREN, 1865

Sa bell idae Fabricia stellaris stellaris (MULLER, 1774) [AI J] ABEH MALMGREN, 1876

W - W - -

-- - -

----!---aw"- ----summ"-

3.2 Dominance

Few species together hold more than 90% of the total abundance: Scoloplos armiger (3 3 %), Pygospio elegans (1 5%), Nereis virens (1 5%), Capitella capitata (1 2%), Microphthalmus s P. (7%), Exogone naidina (3%), Spio martinensis (3%) and Phyllodoce mucosa (2%). 35 species were present with less than 1% of the total abundance.

S, armigerwas the most abundant species throughout the study period (Table 4).

The less abundant species changed ranks between years.

Table 4. Relative abundance of top ranking species in all samples taken in three consecutive years. For Dates See Table 1.

2002 %

5'. armiger 47

Microphthalmus s P. 18

P. elegans 7

A. minuta 6

5'. martinensis 4

P, mucosa 4

2000 O/o

5'. armiger 40 N. virens 30 P. elegans 18 C capitata 6 5'. martinensis 3

2001 %

S. armiger 26

C. capitata 18

P. elegans 13

Microphthalmus s P. 1 2

E. naidina 6

P. mucosa 4

3. 3 Habitat preferences

Comparisons were made first between seagrass beds, cordgrass patches (both as structured habitats) and sandy flats (non-structured habitat).

Juveniles were more abundant in seagrass beds than in sandy flats or cordgrass patches (/72)=7.1151, p<0.05) (Fig. 3, Table 5). Of juveniles, 11 species were present in seagrass beds, 5 in cordgrass patches and 8 in sandy flats. Fabricia stellaris, S. armiger, P. elegans and C. capitata were the dominant species in seagrass beds. S. armiger, C, capitata and

N.

virens had more than 80°/ of the abundance in cordgrass patches. 5, armiger, C, capitata and F. stellaris were in sandy flats the most abundant species.

Seagrass Cordgrass Sandy flats

Figure 3. Mean juvenile abundance in three habitats (August, 2001. Sites F, H in Fig.

1).

*

indicates significantly higher abundances differences (seagrass n= 10, cordgrass patches n=9, sandy flats n=9).

Table 5. Habitat preferences of juveniles. One-way ANOVA test results and Tukey HSD post-hoc values.

One- way ANOVA

I I I I I

Habitat

1

8.480261E+08

1

²

1

^424013040

1

^7.11517

1

^0.003931

I I I I I

Error

1

1.370634E+09

1

²³

1

^59592800

1

Tukey HSD post-hoc test

Also adults were rnore abundant in seagrass beds than in cordgrass patches and sandy flats (F(2i=21.42, p<0.01) (Fig. 4 and Table 6). 13 species were found in seagrass beds, while in cordgrass patches only 6 species were recorded, and 8 in sandy flats. Different species were dominant in each habitat. I n seagrass beds Aphelochaeta marioni, S. armiger, Microphthalmus sp., P. elegans and C. capitata cornprised 80% of total abundance. I n cordgrass patches capitata, Microphthalmus sp. and H. filiformis had together rnore than 80% of total a bundance and in sand y flats C capitata, S. armiger and Microphthalmus sp. were the most abundant species.

seagrass

cordgrass patches Sand

seagrass 0.007528 0.015941

cordgrass patches 0.007528

0.994867

sand 0.015941 0.994867

Seagrass

s

^{Cordgrass San* flats}

Figure 4. Mean adult abundance in three habitats (August, 2001. Sites F, H in Fig. 1).

*

indicates significantly higher abundance (seagrass n= 10, cordgrass patches n=9, sandy flats n=9).

Table 6. Habitat preferences of adults. One-way ANOVA test results and Tukey HSD post-hoc values.

One-way ANOVA

Tukey HSD post-hoc test

A second cornparison was made between mussel beds, fragmented shell patches (both as structured habitats) and sandy flats (as non-structured habitat). I n both, juveniles (/T2)=7.8143, p<0.01) and adults (/ro=10.8745, p<0.01) a significant preference for fragmented shell patches was found (Fig. 5 Tables 7 and 8).

Juvenile mean abundance in mussel beds was significantly lower than in sandy flats and fragmented shells patches (after Tukey HSD-test, p<0.05). 13 species were found as juveniles in sandy flats, where P. elegans, S. martinenis, C.

capitata, Microphthalmus sp. and 5'. armiger had more than 80% of the total abundance. I n fragmented shells patches 12 species were found. Juveniles of 5'.

armiger, C, capitata, P. elegans and Microphthalmus sp. comprised 80% of the total abundance in this habitat. I n mussel beds only 4 species were found as juveniles and only N. virensand S. armigerwere here dominant.

Adult mean abundance was higher in fragmented shell patches than in sandy flats and rnussel beds (after Tukey HSD-test, p<0.01). 17 species were found in sandy flats as well as in fragmented shell patches, and only 7 in mussel beds. I n sandy flats, Malacoceros fuliginosus together with Exogone naidina, P. elegans.

Phyllodoce mucosa and Microphthalmus sp. were the dominant species. I n patches of fragmented shell more than 80% of the total abundance was recorded with E. naidina, Microphthalmus sp. and P.mucosa. I n rnussel beds the dominant species were P. elegans, C capitata and Polydora cikata.

25000

T? .-

U

?j 20000 K

2

15000 C

(U

E 10000

5000

1^1 Juveniles

0 A d u k

Sand Fragmnted Shelfe Mussel beds

Figure 5. Mean adult abundance of juveniles and adults in three habitats (June and August, 2001. Sites B, C in Fig. 1).

*

indicates significantly different abundance cornpared to the other habitats within juveniles and adults (sand n= 6, fragrnented shell patches n=6, rnussel beds n=6)

Table 7. Habitat preferences of juveniles. One-way ANOVA test results and Tukey HSD post-hoc values.

One-way ANOVA

Tukey HSD post-hoc test habitat

Error SS

5.049768E+08 3.231086E+08

Sand

fragmented shell mussel beds

dF 2 10

sand 0.332741 0.041352

fragmentedShell 0.332741 0.007571

P

0.009044 MS

2.524884E+08 3.231086E+07

mussel b e d s 0.041352 0.007571 F

7.81435

Table 8. Habitat preferences of adults. One-way ANOVA test results and Tukey HSD post-hoc values.

One-way ANOVA

1

^sand

1

fragmented shells

1

mussel beds

1

^Sand

1 1

^0.008669

1

^0.543763

fragmented shells

1

^0.008669

1 1

^0.004583

mussel beds

1

^0.543763

1

^0.004583

Tukey HSD post-hoc test

F

10.87456 Habitat

Error

As sandy flats are the most extensive habitat in Königshafen a comparison of three intertidal sandy sites inside the bay was made.

P

0.003100 dF

2 10 SS

1.067135E+09

No significant differences in adult densities were found (/72)=0.379512, p>0.05), while those in juveniles were different (/^5.1221, p<0.01) (Fig. 6 Table 9 and 10). I n Ostfeuerwatt highest mean densities were recorded (after Tukey HSD-test, p<0.05).

MS

5.335675E+08 4.906565E-1-07

Table 9. Mean abundance of juveniles at three sandy intertidal sites in Königshafen One-way ANOVA test results and Tukey HSD post-hoc values.

One-way ANOVA

I I I I I

F 5.12210 locality

P 0.019085 dF

2 SS

52061886 Error

MS 26030943 81313399 16 5082087

Tukey HSD posc-hoc test

1000

0

Oddew att Ostieuewatl Wwenbergwatt Juveniles

Aduits

Figure 6. Mean density  SE of juveniles and adults in sandy flats at three localities in Königshafe (August, 2002. Sites A, B, C in Fig. 1). * indicates significant differences within juveniles (all sites n=7).

Möwenbergwat 0.890558

0.030125 Oddewatt

Ostfeuerwatt Möwenbergwat

Table 10. Mean abundance of adults at three sandy intertidal sites in Königshafen One-way ANOVA test results.

Odewatt 0.047530 0.890558

Ostfeuerwatt 0.047530 0.030125

locality Error

12442151 262276647

2 16

6221076 16392290

0.379512 0.690192

A duster analyzes of the juvenile species composition between sandy flats for August 2002 shows a high similarity between all sites (Fig 7). Sampling sites do not fall into distinct groups.

Figure 7. Cluster diagram of species composition analysis for sandy tidal flats in Königshafen August 2002. (O=Oddewatt, M=Möwe bergwatt, E=Ostfeuerwatt) Index of similarity used: Bray-Curtis.

Comparisons between sub- and intertidal sandy flats for both juvenile and adult polychaetes revealed for juveniles no significant differences (/71,52)=3.8084, p>0.05), while abundances of adults in sub- and intertidal flats were different (^(1,52)=28.7259, p<0.01) (Fig. 8). 13 species were recorded as juveniles in the intertidal sandy flats of Konigshafen, while in the adjacent subtidal flats 17 species were found. I n both, inter- and subtidal flats, 5'. armiger, P. elegans,

C

capitata and 5'. martinensis were the most abundant species. Adults were most abundant in the subtidal flats near Königshafen where also more species were recorded (31 species). I n intertidal flats 25 species were found. P. elegans, S. armiger,

Microphthalrnus sp., C. capitata and M. fuliginosus comprised in both, inter- and subtidal sandy substrata, more than 80% of total abundance.

subtidal nterüda subtidal intertidal

Figure 8. Mean abundance  SE of juvenile and adult polychaetes in both sub- and intertidal flats in Königshafe (August, 2001. Sites A, B, C, and D in Fig. 1).

Black bars=juveniles, white bars= adults (subtidal n=16, intertidal=32).

Only three species were found as larvae at high densities on mussel beds at two times (Table 11). This habitat seems to be an adequate settlement substrate.

Larvae were not or only sporadically present in the benthic samples during this study.

Table 11. Larval densities on mussel beds.

--

^---*""--P

Species Density Date and site P. elegans l1777±777~d/m2~~8,0000

Möwe bergwatt S. rnartinensis 111i10 ind/m2 May 8, 2000.

Möwe bergwatt L. conchilega 583±9 ind/m2 March 3,2001

- - - a - - - - - -

3.4 Species-specific patterns

Patterns of spatial distribution for abundant species were investigated to detect a possible spatial segregation between juveniles and adults with respect to habitat types as well as the intertidal and subtidal Zone.

Scoloplos armiger

Highest juvenile density was 12,760 Â 4,624 ind/m2 and for adults it was 2,719 Â 734 ind/m2. Juveniles were absent in cordgrass patches, while in fragmented shell patches abundances were significantly higher than in sandy flats and seagrass beds ( / 7 2 ) = 27.98677, p<O.Ol).

Juveniles showed similar dynamics at inter- and subtidal zones. Density increased continuously from March to August in both environments (Fig. 9). Adults showed an even temporal pattern over the Same period.

S.

armiger juvenile and adults share the Same habitat through time.

W

s March May Juty A u g

March May Juty A u g

March May l u i y A u g

March May Juiy A u g

Figure 9. Temporal variability of juvenile and adult Scofopfos armiger in 2001 in Königshafe (sites A, B, C, E and D in Fig. 1 respectively) (black bars=juvenile, white bars= adults).

Nereis virens

Juvenile densities up to 12,796 Â 4,875 ind/m2 were observed. Juveniles of N.

virens were absent in seagrass beds but no significant differences in mean densities occurred between habitats in the intertidal Zone (F(2)=2.3668 p>0.5).

Juveniles of N. virens were most abundant in the intertidal Zone but were present also in the subtidal. Adult densities were similar between intertidal and subtidal (F(2)=2.4723, p>0.05). No segregation Patterns between juvenile and adult worms were found, but i t may be possible that intertidal juveniles migrate to the subtidal in order to maintain this adult population at the Same density as in the intertidal (Fig. 10)

INTERTIDAL SUBnDAL

700 700

600 600

500 500

400 400

300 300

201 200

NE

^{100 100}

? ^{0 0}

a hbrch May Aug hby Juty Aug

Å

6 700 -0

4

⁶⁰⁰ : ⁵⁰⁰

^

⁴⁰⁰

300 300

200 200

100 100

0 March hlay Aug

:L

^{h6ay Juiy Aug}

Figure 10. Temporal abundante variability in 2001 of N. virens at inter- and subtidal fiats in Königshafe (sites A, B, C, E and D in Fig. 1 respectively) (black bars=juveniles, white bars=adults).

Pygospio elegans

The highest density of juvenile P. elegans was 7,419 Â 983 ind/m2 and up to 3,809 Â 1,574 ind/m2 adults were recorded. Post-setiled larvae were found only in the subtidal Zone (475 Â 23 ind/m2).

Fragmented shell patches were the most suitable habitat for juvenile P, elegans (F(3)=149.9072, p<0.01). And for adults sandy flats were the most adequate substrata (F(3)=3.4073, p<0.05). This may indicate a juvenile migration to the surrounding sandy habitats.

The temporal variability of juvenile and adult abundances showed no differential patterns between tidal zones (Fig. 11).

INTERTIDAL 4000

3500

\ ^T

C, M a c h May Juiy Aug

W "

6 ⁴⁰⁰⁰ 5 3500

-s

³⁰⁰⁰

[

Maich May July Aug

imOi,

⁵⁰⁰

_-

^,

_?::;L7

0

March May Juiy Aug M a c h May Juty

Figure 11. Temporal variability in 2001 of juvenile and adult P. elegansin Königshafe (sites A, B, C, E and D in Fig. 1 respectively) (black bars=juvenile, white bars= adults).

Ophelia rathkei

Juveniles were present in sandy flats and fragmented shell patches, with no significant differences in abundance (F(2)=0.0033, p>0.5). Juveniles were present only at the intertidal Zone with densities up to 125*71 ind/m2. This represents a spatial segregation between adults and juveniles (Fig. 12).

INTERTIDAL SUBTIDAL

400 350 300

250 250

200

150 150

-

U^ 100 100

"E 50 50

2 _{d 0}

G 8 ^- .

:L

^{March May July Aug}

300

2 250 250

200

150 150

100 100

50 50

0 0

Mrch May July March M y July Aug

Figure 12. Temporal abundance variation of juvenile and adult 0. rathkei in inter and subtidal regions around Königshafe (sites A, B, C, E and D in Fig. 1 respectively) (black bars=juveniles, white bars=adults).

Microphthalmus s P.

Maximal abundance of juveniles was 2,987 Â 984 ind/m2, of adults 16,171

*

7,273 ind/m2. Juveniles of Microphthalmus sp. were only found in sandy substrata and among fragmented shells but no significant differences were found (Fm=1.1147, p>0.05).

The temporal abundance variation showed that juveniles were mainly at the intertidal, while adults occurred in both zones. The adult peak in August (Fig. 13) may stem from intertidal juveniles which migrated to the subtidal.

INERTIDAL SUBTlDAL

8

_ Â ¥

^{, , - - - ,}

2 1000

.

^V _.- ^{t- 0}

U _0, March May July Aug Maich May July Aug

March May July Aug March May July Aug

Figure 13. Temporal abundance variation in 2001 of juveniles and adults o f Microphthalmus sp. in Königshafe (sites A, B, C, E and D in Fig. 1 respectively) (black bars=juveniles, white bars=adults).

Phyllodoce mucosa

As in Microphthalmus sp. and 0. rathkei juvenile P. mucosa were only found in sandy flats and among fragmented shell patches but no significant differences occurred (Fcn=0.65930, p>0.05). Juveniles were often found in L. conchilega tube-mats in the subtidal Zone (per. Observ.). Adults were present in the intertidal Zone as well as in the subtidal. Juveniles had highest abundances in the subtidal from July to August. I n the intertidal, only in July an abundance peak was observed (167*27 ind/m2). Juveniles from the subtidal may supply the intertidal part of the adult population (Fig. 14).

- -

U .-

a, March M a y Juiy Aug March May Juiy Aug

800

A o

^{March May July Aug 700 600 500 400 300 200 100}

Figure 14. Temporal abundance variability in 2001 of juvenile and adult P. mucosa at the intertidal and subtidal around Königshafe (sites A, B, C, E and D in Fig.

1 respectively) (black bars=juvenile, white bars=adults).

Discussion

Species spectrum and dominance

Hundred-thirteen polychaete species have been registered for the Northern Wadden Sea, in larval, adult and juvenile stages (reviewed in Rodriguez-Valencia 2003). The present study detected 41. The absence of some species may be explained by a limited sampling effort over habitat types and seasons. Species which are present only as larvae were not taken into account. The identification of juveniles based on identification keys for larvae and adults may cause errors in identification. Another factor generated this low number of species here found, could be the high fluctuation in benthic composition due to climatic factors (Strasser etal. 2001 a and b, Strasser and Pieloth 2001). Westheide (1966) record 45 polychaete species for the study area, most of them mainly present in sandy beaches, this habitat was not sampled in this study. The dominance of S. arrniger, P, elegans, and C capitata over various habitat types in the study area is well

known (Reise 1983, Reise et al. 1994). Nevertheless, the lack of samples from muddy substrates in this study, let to a low degree of dominance in some other species that are also important components of the Wadden Sea fauna (e.g.

Heteromastus filiforrnis a nd Tharyx s p

.

⁾

.

Habitat preferences

Sandy sediments Cover about 72% of the intertidal and predominate also the subtidal Zone in the List tidal basin (Bayerl et al. 1998). Polychaete biomass in these areas is dominated by Arenicola marina (Reise and Lackschewitz 1998).

Adult and juvenile polychaete densities were lower in sand in contrast to in seagrass beds. This may be explained with sandy flats being a less structured ha bitat, and sedi ment instability and high exposure to predators. Some differences in juvenile abundance between sandy areas inside Königshafe were found, although the sediment composition seems to be similar. The northern most part (Ostfeuerwatt) was characterized as an erosion area (Higelke 1998), and here juveniles were most abundant. This may be due the common occurrence of fragmented shells at eroding flats.

The intertidal Zone of the List tidal basin is covered up to 12% with seagrass.

Meadows are located in areas protected from westerly storms and are more or less stable (Reise and Lackschewitz 1998, Schanz 2003). Abundances of juvenile worms were higher in seagrass beds than in other habitats. They may offer a more structured habitat than the sandy flats (Bell et al. 1992, Valentine and Heck 1990, Mantilla etal. 1999, Boströ and Bonsdorff 1997 and 2000). Results of this study show that seagrass beds were a preferred habitat for juvenile and adult polychaete worms.

Cordgrass patches were poorly colonized by juveniles and adults. Cordgrass patches are a relatively new habitat. The species was introduced in the 1920s in the German Wadden Sea as a land reclamation measure and its distribution is still expanding (Löb 2002). Löb (2002) found that only juvenile Arenicola marina were present between stems of S. anglica. Despite the highly structured habitat,

patches of S. anglica offer no nursery function for other polychaete species probably due to the position in the upper intertidal with a short immersion time.

Intertidal rnussel beds represent about 1% of the basin's surface (Saier 2000).

Only P. ciliata as adult seems to be representative for mussel beds, while its juveniles are also present in other habitats (pers. obser.). Since larvae of P.

elegans, S. martinensis and L. conchilega were found only in mussel beds, it is possible that these beds act as larval traps or settlement substrata but are unsuitable for adults. Adults of all three species are common in sandy substrata (Hartmann-Schröde 1996). Active or passive secondary dispersal may have occurred in this case.

Fragmented shell patches are considered as a temporary dynamic habitat but the area covered is not known (Wolf 2002). Here both adults and juveniles had a higher density than in rnussel beds or sandy flats. Wolf (2002) could not find a significant difference between densities of infauna in both fragrnented shell patches and sandy sediments, but found a higher abundance of adults from M.

fuljginosus, P, mucosa and Polydora spp. compared to ambient sand flat areas.

From the above it rnay be concluded that structured habitats like seagrass beds and fragmented shell patches are habitats preferred by juvenile and adult polychaetes over unstructured sandy bottoms. Other structured habitats were not favorable. Cordgrass patches may be to high in the intertidal Zone and in mussel beds the sediments may be too anoxic to attract polychaetes. With respect to tidal zones juvenile polychaetes dominate the intertidal and adults dominate subtidally.

Species-specific patterns

For many benthic marine invertebrates extensive dispersal is assumed to occur primarily during the planktonic larval stage over large distances (Strathmann 1974, Scheltema 1986), whereas dispersal during the post-settlement juvenile and adult stages is thought to be less important (Norkko et al. 2001). Nevertheless, the spatial Segregation between juvenile and adults of some species reported here, suggest that polychaetes are able to undertake some migrations. I n mussels

the so-called bysso-pelagic migration phase (Bayne, 1964) was observed.

Arrnonies (1992, 1994 and 1999) describes the drifting of meio- and macrobenthic invertebrates on tidal flats in the study area. He concludes that active Initiation of drifting may occur: a) by individuals in order to escape from an unexpected threat, b) by group evasion as a reaction to factors accumulating over the time or C) as a habitat change in the Course of development.

I n polychaetes the best studied case of juvenile segregation and their subsequent migration is for Arenicola marina (Farke and Berghuis 1979, Farke et al. 1979).

But also Armanda amakusaensis (Tamaki 1985) presents this phenornenon. I n this study A. marina was not considered. The clearest case found was 0. rathkei, I t has shown juveniles spatially separated from the adults, but this may be due an inadequate sampling of coarse sand in the intertidal. Reise (1982) found adult 0.

rathkeiin the beach while juveniles occurred On the tidal flat.

Post-settlement movernents by intertidal benthic macroinvertebrates seem to be a common event (Cummings etal, 1995) that enable organism to respond to habitat patches of different quality (Hastings 1990, Possingham and Roughgarden 1990).

For the maintenance of populations in estuarine habitats, dispersal of young benthic Stages seems to be important (Daunys et al. 2000, Essink and Dekker 2002).

N, virens is commonly present from the upper intertidal to 150 m depth in the subtidal Zone (Hartmann-Schröde 1996). I n the study area it was found to be mainly restricted to the subtidal and the lower intertidal (Reise pers. comm.).

During this study it was observed that the distribution reaches the high tide line.

For S. armiger no segregation between adults and juveniles was recorded, but it was considered as only one species with two different reproductive modes (pelagic larvae and direct development). Now two different species for the genus Scoloplos in the study area are proposed (Kruse et als 2003, Albrecht 2004). Juvenile Scoloplos spp. are capable to migrate in the water column (Armonies 1999), but it is unknown if these movements may take a specific direction. Reise (1987) reports the lower, seaward flats as the preferential habitat of juvenile S. armiger.

The question to answer is: are tidal flats in the List tidal basin a nursery habitat for polychaetes? According to this study, seagrass beds as well as fragmented shells patches retain a large number of juvenile polychaeta (e.g. Scoloplos armiger and Pygospio elegans). I n Phyllodoce rnucosa the subtidal beds of Lanice conchilega may qualify as a nursery.

The importance of some habitats as nurseries has been widely discussed (Boesch and Turner 1984, Robertson and Blaber 1992, Primavera 1998). I n the Wadden Sea mussel beds were described as nurseries for the periwinkle Littorina littorea (Saier 2000), and the intertidal flats for juveniles of Cmgon crangon (Cattrijsse e t al, 1997) and for Macorna baltica due the low predation pressure in this habitat (Hiddink etal. 2002).

Beck e t al. (2003) proposed that a near-shore habitat serves as nursery for juveniles of a particular fish or invertebrate species, if it contributes disproportionately to the size and numbers of adults relative to other juvenile habitats. The disproportionate contribution to the production of adults can come from any combination of four factors: density, growth, and survival of juvenile animals, and their movement to adult habitats.